1. Field of the Invention
The invention relates to the packaging of integrated circuits for use with microwave and millimeter wave signals. The invention relates more particularly to the packaging of hybrid interconnected integrated circuits, typically of a monolithic microwave construction which incorporates passive and active components, i.e. "MMIC" chips, and the packaging of such MMIC chips in a sealed enclosure, typically hermetically sealed.
2. Description of the Prior Art
Monolithic microwave integrated circuit (MMIC) technology has proven useful in electronic circuitry operating at frequencies in the Gigahertz range. Currently, the MMIC technology is in use in frequencies from a Gigahertz to frequencies above 100 Gigahertz. A preferred material for the individual chips is one of the preferred high frequency semiconductor materials such as gallium arsenide. In fabricating the chips both passive and active circuit elements are photolithographically patterned. Because of a variety of fabrication and performance problems, it is usually most convenient to make up higher level circuits or "modules" from a plurality of interconnected MMIC chips thus creating a "hybrid" integrated circuit assembly. A common module is a microwave transmit/receive module for use in phased array systems in use in satellite communication and in radar systems.
For performance reasons, each module should contain all the circuitry directly associated with an antenna element and required for processing signals at the transmitted frequencies. Within the modules, which tend to become quite small at the higher frequencies, the individual chips should be interconnected by connections which preserve transmission line quality (i.e. maintain transmission line impedances and avoid reflection causing discontinuities) and which are short to minimize time delays in processing the signal. Since the MMIC chips are frequently subject to digital phase and amplitude controls, a collection of interconnected MMIC chips making up a module may require a large number of interconnections which, as the module sizes go down, become more and more closely spaced.
The high density interconnection (HDI) technique described in two patents assigned to the Assignee of the present application (U.S. Pat. No. 4,783,695, filed Sept. 26, 1986 entitled "Multichip Integrated Circuit Packaging Configuration and Method"/C. W. Eichelberger and R. J. Wojnarowski and U. S. Pat. No. 4,894,115, filed Feb. 14, 1989 and entitled "Laser Beam Scanning Method for Forming Via Holes in Polymer Materials"/C. W. Eichelberger, R. J. Wojnarowski and K. B. Welles), describes a method of interconnecting MMIC chips requiring the high density of connections adequate to meet the need in such modules.
The HDI process, however, conventionally uses a substrate of alumina supporting MMIC chips of gallium arsenide. In the hybrid assembly, the chips are supported in the alumina substrates with their upper surfaces and with the upper surface of the substrate flush. Then a metallizable dielectric layer in the form of a thin (1 mil) solid sheet is adhered to the flush surfaces. The dielectric layer bridges small gaps in the underlying surface and accommodates small variations in the height of the surfaces. The chips and substrate are electroded prior to the application of the dielectric, and after the dielectric layer is applied "via" holes are etched down through the dielectric to these electrodes. A patterned metallization, as described in the cited patents, is then formed on the dielectric layer which only connects the pads exposed at vias.
The problem of achieving the flush surfaces in the known process is solved in an alumina substrate by a machining process or by casting in recesses, or by inserting shims under the chips, etc. Machining is a very difficult process since the material is quite hard, granular and fragile in thin sections. While other materials, such as silicon, may have been suggested as providing a suitable substrate for use with the high density method of hybrid interconnection, no realizations of hybrid circuits using a silicon substrate are known nor are any fully enclosed or hermetically sealed assemblies for hybrid interconnected MMICs known using a silicon substrate.
In addition to the fabrication problem associated with achieving flush surfaces, alumina material, which is both granular and opaque, is not readily adapted to receive fiber optic connections for signal or control purposes. Such connections are of increasing interest where high density interconnections are required.